1. Field of the Invention
The invention relates to an improved process for the production of the fragrance trans-3,3,5-trimethylcyclohexyl ethyl ether by hydrogenation of the enol ether 3,3,5-trimethylcyclohexenyl ethyl ether in a high yield and with high selectivity under particularly mild reaction conditions.
2. Statement of Related Art
The hydrogenation of enol ethers is basically known from the literature. For example, A. Yanagawa, Y. Suzuki, I. Anazawa, Y. Takagi and S. Yada hydrogenated methyl-4-tert.butyl-1-cyclohexenyl ether. They obtained mainly the cis-isomer when the catalyst contained metallic rhodium and mainly the trans-isomer when the catalyst contained a rhodium complex (J. Mol. Catal. 1985 (29) 41).
S. Nishimura, M. Katagiri, T. Watanabe and M. Uramoto have shown that the Pd-catalyzed hydrogenation of the enol ethers of 2- and 4-methyl cyclohexanone leads with high stereoselectivity to the saturated cis-ethers (Bull. Chem. Soc. Jpn. 1971 (44) 166).
In another work, S. Nishimura, K. Kagawa and N. Sato (Bull. Chem. Soc. Jpn. 1978 (51) 3330) showed that, in Pd-catalyzed hydrogenation in ethanol, the corresponding isomeric enol ethers of 3-methyl cyclohexanone, 1-ethoxy-3-methyl and 1-ethoxy-5-methyl cyclohexene lead mainly to the trans-ethers in the initial phase of the reaction. However, the cis components were still 23% and 15%. In addition, the reaction has the disadvantage that it has to be carried out in a solvent. On the one hand, this unnecessarily complicates working up of the product; on the other hand, it can give rise to the formation of secondary products.
According to the teaching of DE 26 61 006 C2, 3,3,5-trimethylcyclohexyl ethyl ether (1) can be obtained in a yield of 75% of the theoretical by nickel-catalyzed hydrogenation of 3,3,5-trimethylcyclohexenyl ethyl ether ("enol ether"). The required trans-form of the ether is formed with 95% selectivity. However, energy-intensive temperature and pressure conditions are required for this reaction. According to the claims of the document in question, the reaction is carried out*at temperatures of 150.degree. to 200.degree. C. and under hydrogen pressures of 10 to 200 bar. ##STR1##
In order to reduce energy consumption and the potential dangers involved in handling hydrogen gas under such conditions and to increase the life of the reactors, a search was made for a process to produce (1) which could be carried out under much milder conditions for at least the same selectivity. In addition, for reasons of economy, the yield obtained from the hydrogenation of the enol ether would be improved for low concentrations of catalyst. Finally, the process would be carried out under conditions which made the presence of a solvent unnecessary.